Compositing node not showing up in render
One way is to create two render layers and then mix them in the compositor. For instance



Here is how I made my example


Create the entire scene on layer 1 as usual. For the example I placed a sphere in front of a cube.
Move the object that you want the glow on to another layer.  For the example I moved the cube to layer 2.
In properties -> Scene modify the 'Scene' layers to contain both layers and 'Layer' layers to contain only layer 1

Add a new Render Layer.  Being lazy I left its name as "RenderLayer.001". Adjust its 'Layer' layers to contain only layer 2 and disable 'Use Environment'